THE PAEATHYEOID GLAND AND ITS DISEASES 503 



Embryology 



Origin. The parathyroid glands develop as thickenings of the endo- 

 derm of the third and fourth branchial clefts. These clefts are illustrated 

 in Fig. 2, as seen from the inner surface of the pharynx of a human 

 embryo. Their derivatives are indicated diagrammatically in Fig. 3. The 

 superior parathyroids (marked with vertical lines) develop from the 



Fig. 3. Diagram showing derivatives of branchial pouches: thyroid (t) and 

 ultimobranchial bodies (u) in horizontal lines, parathyroids (p) in vertical lines 

 and the thymus (th) dotted, after Grosser, modified. 



fourth cleft and are for this reason sometimes termed the glandulse para- 

 thyroidese IV. The inferior parathyroids (marked in the same way) 

 arise from the third cleft and are called the glandulse parathyroidese III. 



It will be seen that there has been a reversal, for the superior para- 

 thyroids (IV) are developed from the fourth pouch, yet they are located 

 anteriorly to the glandule parathyroidese III. This has been brought 

 about by the fact that in the general caudal migration of the branchial 

 derivatives, the thyroid and the glandulse parathyroidese IV have been 

 outdistanced. 



Growth. The growth phenomena of the parathyroid glands have not 

 been carefully studied, even in animals. Sexual variations are unknown 

 in man. The glands appear, however, to be relatively larger in female 



